1. Field of the Invention
This invention relates generally to a rotating cylindrical magnetron apparatus, and more specifically to an improved method and apparatus for affixing the target of such an apparatus to its support spindle.
2. Description of the Prior Art
This invention relates directly to an apparatus used for magnetron sputtering utilizing rotating, cylindrical shaped targets. The cylindrical targets are used to deposit various materials onto a variety of substrates, typically, but not limited to, glass substrates. The materials are deposited in a vacuum chamber using a reactive gas, magnetron sputtering technique. The principles and operation of one such apparatus are described in detail in Boozenny et al. U.S. Pat. No. 5,096,562, describing a large-scale cylindrical sputtering apparatus used for sputtering coatings on architectural glass panels, automobile windshields, and the like.
There are several advantages for using a rotating cathode apparatus, as opposed to a flat magnetron system. Some of the advantages include higher deposition rates, higher target material inventory (utilization) per cathode, low maintenance frequency and better cost efficiency, all as compared to a planar magnetron.
Another major advantage for using a cylindrical target, as opposed to a planar target, is that a rotating cylindrical target can successfully sputter silicon compounds, for example Si.sub.3 N.sub.4 and SiO.sub.2. These compounds are extremely difficult to produce using a planar target, as is taught by Wolfe et al. U.S. Pat. No. 5,047,131.
The foregoing are advantages in principle, but have not been proven to be advantages in fact. While a considerable amount of effort has been spent developing the apparatus used for rotating cylindrical targets, the advantages have not been demonstrated in practice. The apparatus which has been used for sputtering with cylindrical cathodes on large scale substrates has failed to demonstrate most of the above-noted advantages, with the exception of being able to sputter Si using reactive gases used to form Si compounds. However, while it has been possible to sputter reactive Si compounds, the reliability issues related to the cylindrical target apparatus overshadow the advantages.
The cylindrical target apparatus, as described in Boozenny et al., prohibits the possible advantages from being achieved, due to frequent mechanical break-downs resulting in significant loss of production time. A common source for mechanical failures of the rotatable cathode are water leaks occurring at the interface of the support spindle and cylindrical target. In order to make the rotatable cathode efficient and to accommodate the required cooling and magnet assemblies, it is necessary that a detachable cylindrical support spindle be employed in the design to support the target structure on at least one end of the target. The reason for employing a detachable support spindle is to allow access and removal of the cooling and magnet assemblies during target replacement. Further, providing a detachable spindle mechanism provides a more efficient method for the manufacturing of the cylindrical targets.
A major consideration in the design of a detachable support spindle is that this interface represents one of the most critical sealing areas of the cathode system. This is due to the fact that any seal at the spindle-to-target interface is subjected to high vacuum on one side of the seal surface, and high water pressure on the opposite side of the seal surface, representing an extreme condition for a seal.
Another consideration for a detachable support spindle is the number of seals that are utilized in the design. Therefore, it becomes most desirable to minimize the sealing surface to a single seal, thereby minimizing the possible locations for water-to-vacuum leaks within the vacuum chamber to a single location, as opposed to multiple locations.
Yet another consideration for the spindle-to-target interface is where and how the seal is placed with respect to the detachable support spindle and the target's sealing surface(s). To minimize the possibility of water-to vacuum leaks within the vacuum chamber it is advisable to place the seal in an area of this interface where the sealing surfaces are least likely to be damaged during the handling, cleaning and replacement of the target structure. Therefore, placement of the seal at the end of the target, on a flat surface, perpendicular to the target surface, should be avoided. Also, the seal itself should be prevented from any movement during the process of attaching the spindle to the target, and that during this process the seal should be compressed in a uniform manor, so that an equal and continuous force is exerted during the compression of the seal.
Still another consideration for the detachable support spindle is to provide a quick and easy methodology for removing the detachable support spindle from the target during the process of maintenance, repair, and replacement of the target. However, the spindle-to-target interface can be subjected to extremely high axial and radial forces. These high forces can be created by a combination of factors. The length, weight, rigidity and concentricity of the cylindrical target account for some the factors that contribute to the high forces involved. Another contributing factor is the high temperature that is applied to the target during the sputtering process, causing thermal expansion of the target, both longitudinally and radially. It is therefore not advisable to incorporate any clamping structures to the inside circumference of the cylindrical target for the purpose of affixing the support spindle structure to the target structure, and/or for the purpose of compressing the seal at the spindle-to-target interface.
Yet another important factor for consideration is that the support spindle must accommodate the rotation of the target in either a clockwise or counter-clockwise direction during the sputtering process. The methodology employed for a quick and easy removal system should comply with all of the above mentioned considerations.
Another consideration for a detachable support spindle is that the attachment methodology should allow for an efficient and uncomplicated means of recycling the support cylinders that are often used to carry the target materials used for sputtering. For example, in some cases a cylindrical support structure must be utilized for the sputtering of materials that do not have enough mechanical strength to support themselves (e.g., Silicon or other materials that are either too soft or too brittle). From an efficiency standpoint, the support structures should have the capability to be recycled. Therefore, it becomes important to consider the method of attaching the support spindle to such a cylinder.